1. Field of the Invention
The present invention relates to a remaining battery capacity measuring method and apparatus, and more specifically to the method and apparatus for displaying a remaining battery capacity precisely, in particular when there exists no strong correlation between dispersive voltages and dispersive currents of a battery to be measured.
2. Description of the Prior Art
In general, there exists a negative correlation between the dispersive voltages and the dispersive currents of a battery. In other words, when the discharge current is small, the battery voltage is high; and when the discharge current is large, the battery voltage is low, in general. Further, when the battery capacity is low, the battery voltage drops more at a constant current, as compared when the battery capacity is high.
By utilizing the above-mentioned battery discharge characteristics, a remaining battery capacity can be estimated, as disclosed in Japanese Patent Application No. (6-200953, for instance. In this prior art method, for instance, dispersive battery voltages and dispersive battery currents flowing through various loads are sampled for each predetermined time interval; the sampled voltage and current data are averaged for each predetermined time interval; when a predetermined number of average voltage and current data have been obtained respectively, an approximate linear function (equation) is calculated on the basis of the average voltage and current data and in accordance with the least square method and further a correlation coefficient is calculated on the basis of the average voltage and current data; when the calculated correlation coefficient is smaller than a negative reference value (i.e., a strong correlation is obtained), a voltage corresponding to a predetermined discharge current is calculated on the basis of the approximate linear Function; and finally a remaining battery capacity corresponding to the voltage is estimated and displayed.
In summary, on the basis of the fact there exists a negative correlation (e.g., less than -0.9) between the dispersive voltages and currents of the battery, the approximate linear function is calculated by use of the average dispersive voltage and current data of the battery, and the remaining battery capacity is estimated on the basis of the calculated approximate linear function.
In the prior art remaining battery capacity measuring method, however, a problem arises when this measuring apparatus is mounted on an electric vehicle to measure the remaining capacity of a traction battery, for instance. In more detail, since the electric vehicle is driven by a motor mounted thereon by use of the battery energy, when the electric vehicle is running on a descending road and therefore the accelerator is not depressed by the driver, the motor generates a regenerative current as a generator, so that the traction battery is reversely charged by this regenerative current of the motor. In this case, although the power reduction of the traction battery can be prevented, there exists a tendency that the voltage rises gradually due to a charge by the regenerative current of the motor.
Further, in this case, although it is possible to detect the regenerative current to disable the measuring apparatus in such an unusual state, since the regenerative current must be detected by use of a complicated detection system, the detection system becomes costly, so that another detection system is not adopted usually.
Further, when the electric vehicle is left stopped, since the traction battery is not consumed, the battery voltages and currents hardly change. Under these conditions, in spite of the fact that the correlation is not established between the voltages and currents of the traction battery, when the approximate linear function of the voltages and currents of the battery is calculated and further the correlation is determined as being less than a reference value erroneously, the remaining battery capacity is also estimated erroneously.
In other words, in the prior art remaining battery capacity measuring method as described above, once the regenerative current is being generated by the motor or when the vehicle is left stopped, the definition of the detected battery current is not clear. As a result, when the approximate linear function between the currents and voltages of the battery is calculated on the basis of this indefinite voltages and currents, a measurement error is inevitably produced, thus deteriorating the reliability of the remaining battery capacity measuring apparatus.